In general, compositions for use in the preparation of positive-type light-sensitive lithographic printing plates are those containing an o-quinonediazide compound as a light-sensitive constituent and an alkali-soluble resin for increaseing the strength of the coat layer on the plate.
Novolak resins such as phenol-formaldehyde resin, cresol-formaldehyde resin, etc., have conventionally been used as the alkali-soluble resin. West German OLS Patent No. 2,616,992 describes the use of a novolak resin, as a binder, obtained by the polycondensation of a phenol substituted by an alkyl group having from 1 to 9 carbon atoms and formaldehyde to improve the printing plate on the resistance thereof against alkaline developer and on the wear resistance of the light-sensitive layer thereof. Further, Japanese Patent Publication Open to Public Inspection (hereinafter referred to as Japanese Patent O.P.I. Publication) No. 127553/1980 describes the use of a copolycondensed novolak resin, as a binder, obtained by the condensation of at least one alkyl-phenol and phenol or a methyl-substituted phenol or a mixture of these and formaldehyde to improve the resistance against an alkaline developer like above.
The above two resins, however, are poor in the alkali-solubility. A lithographic printing plate that uses a lightsensitive composition containing these resins as binder materials has the disadvantage tht its alkali-solubility at the time of its development is bad; not satisfactorily developed when processed in an exhausted alkaline developer solution; and thus the sensitivity of the printing plate becomes deteriorated.
Japanese Patent O.P.I. Publication No. 57841/1980 describes an alkali-solubility-improved novolak resin obtained by the co-polycondensation of phenol and cresol with an aldehyde. However, the resin, although satisfactory in the alkali-solubility, is not necessarily sufficient in the resistance to processing chemicals. A positive-type light-sensitive lithographic printing plate, when it undergoes a plate-making treatment under a white fluorescent lamp light, produces a fog to cause a loss of the light-sensitive layer in the image area during its development process, thus leading to deterioration of the printing plate's press life (such characteristics are hereinafter called "safety-to-light characteristics"). The use of the above-described co-polycondensed resins is disadvantageous in that it increases such deterioration of the press life due to fog. The foregoing co-polycondensed resins, if their weight average molecular weight exceeds 6000, will improve the printing plate's processing chemical-resistant characteristic, but on the other hand there occurs a contradiction that their alkali-solubility and sensitivity become deteriorated at the same time.
Further, Japanese Patent Examined Publication No. 23570/1979 describes the improvement of the printing plate on the sensitivity thereof by use of two types of phenol-formaldehyde novolak resins or resol resins different in the solubility to an aqueous alkaline solution having a pH of not more than 12 as the binder resin to be contained in a photoresist composition. However, since a developer solution for use in processing the positive-type light-sensitive lithographic printing plate has a high alkalinity (pH approximately 13), in the case where the foregoing two types of resins are used as the binder for the light-sensitive composition of the positive-type light-sensitive lithographic printing plate, the sensitivity of the plate can be improved, but there occurs an extreme corrosion in the unexposed image portion of the light-sensitive layer by an alkaline developer liquid used for its development, resulting in the deterioration of the ink receptivity, processing chemical-resistant characteristic, and safety-to-light characteristics of the printing plate.
Japanese Patent O.P.I. Publication Nos. 101833/1982 and 101834/1982 describe a method for improving the processing chemical-resistant characteristic, alkali-solubility, and press life of the printing plate by use of a polyhydric phenolbenzaldehyde condensate such as resorcinol-benzaldehyde resin, pyrogallol-benzaldehyde resin, etc.; a polyhydric phenol-acetone co-polycondensate resin such as pyrogallol-resorcinol-acetone resin, etc.; and the like, as the binder for the light-sensitive composition of a positive-type light-sensitive lithographic printing plate. This method, however, is disadvantageous with respect to the safety-to-light characteristics.